1. Field of Invention
The present invention relates to a video signal capturing apparatus for capturing video data for use by an external processing apparatus by converting input video signals into digital video data and storing the digital video data in a memory so that the digital video data may be used by the external processing apparatus.
2. Description of Related Art
Many systems have been developed in which input video signals are image-processed in a predetermined manner by a personal computer or the like or printed out by a printing mechanism. Such a system employs a video signal capturing apparatus for capturing video signals to be used by an external processing apparatus, for example, a personal computer or a printing mechanism.
Video signals systems such as a NTSC system use a four-field sequence as described below. An NTSC system employs an interlace scanning method in which a frame is formed by two vertical sweeps and 525 horizontal sweeps. Interleaving is performed to reduce the influence of chroma components on luminance components. That is, a frequency of the color subcarrier waves (i.e., the subcarrier wave for the chrominance signals) is selected such that the multiple product of an odd number and the half period of the subcarrier waves equals the horizontal sweep time duration. Therefore, the color subcarrier waves are completely out of the phase, i.e., inverted 180.degree., for every horizontal scanning line. Since the total number of horizontal lines scanned for a single frame by two fields, is 525, which is an odd number, the phase of the color subcarrier waves is opposite for every frame. More specifically, the polarity of the sine waveform at a given horizontal position on each scanning line alternates for every frame. Since the total number of horizontal lines scanned for two frames by four fields is 1050, which is an even number, the color subcarrier waves and the horizontal lines are in phase every two frames. Thus, taking multiplex chrominance signals into account, the video signals of the NTSC system are in four-field sequences in which a single sequence, including the phase of the color subcarrier waves (frequency fsc=3.58 MHz), is completed by four fields even in the case of a static picture image. The four fields are termed color fields 1 to 4, respectively.
Some conventional video signal capturing apparatuses capture video signals by converting input video signals into digital video data and storing the digital video data in a memory. In a video signal capturing apparatus of this type, video signals which include horizontal synchronizing signals may be captured by analog-to-digital conversion as indicated in FIGS. 3-5. Although the time relationship between the horizontal synchronizing signals and all the data is indicated in the captured digital data, the captured data contain unnecessary data as well because all the data, including data from portions where no image information is carried, are captured.
As a countermeasure, only the digital video data from portions that carry image information may be captured as indicated in FIG. 4. However, this method has problems if a horizontal synchronizing signal is disturbed. For example, when video signals to be captured are provided from a storage medium, e.g., a video tape, optical (magnetic) disk or the like, horizontal synchronizing signals may be lengthened or shortened. Therefore, the horizontal synchronizing signals are shifted from a predetermined timing on the time axis by influence from a drive mechanism that is necessary to drive or turn the storage medium for reproduction. When the horizontal synchronizing signals are thus disturbed, the time intervals between scanning lines become inconsistent resulting in indeterminable time intervals. That is, if data capture is performed with reference to horizontal synchronizing signals, a disturbance in the horizontal synchronizing signals makes the time relationship between video data of sequential horizontal scanning lines indeterminable. With such an indeterminable timing in the captured digital video data, an external processing apparatus cannot properly processes the digital video data.
The aforementioned problem of a timing shift of horizontal synchronizing signals can be solved by separating brightness signals Y and chrominance signals C beforehand by a YC separating method, and capturing the separated signals by converting them separately into digital data. In this case, however, the apparatus structure becomes complicated because a YC separating circuit for separating brightness signals Y and chrominance signals C beforehand and A/D converters for converting the brightness signals Y and the chrominance signals C separately into digital data are required. Furthermore, in a circuit performing separated data capture, it is common practice to employ a YUV data capturing method in which the chrominance signals C are further separated into horizontal frequency components U and vertical frequency components V. If YUV data capturing is employed, circuit structure for the further separation and an arrangement for the A/D conversion of the separated signals become complicated.